Caustic soda is employed by a wide range of industries as building blocks for such commonly used products as plastics, bleach, flexible and rigid foams, pharmaceuticals, herbicides, paper products, and for water purification and neutralization. Caustic soda (NaOH, lye), or sodium hydroxide, is produced simultaneously with chlorine, usually through the electrolytic decomposition of sodium chloride solutions in diaphragm, membrane, or mercury cells.
In most applications, caustic is sold and used as a 50% solution in water. It is valued more for its neutralizing power as a strong base and as an absorbent than as a source of sodium. End-uses for caustic soda include pulp and paper, soaps and detergents, aluminum, petroleum, natural gas refining and processing, and water treatment and cotton processing. It is also used to produce a wide range of organic and inorganic chemicals.
The most inexpensive source of caustic soda is from chlor-alkali plants. The feed material for chlor-alkali cells is sodium chloride solution. Chlorine is produced as an inevitable by-product of the chlor-alkali process, at a rate of 1.0 to 1.1, usually through the electrolytic decomposition of salt solutions in electrolytic diaphragm, membrane or mercury cells. This chlorine by-product has been used by pulp mills as a bleaching agent since the turn of the century. Recently, it has been found that chlorine bleaching produces highly hazardous organochlorine compounds. Environmental protection agencies have therefore prohibited the use of chlorine for pulp bleaching.
Caustic soda producers have attempted to find alternative technologies for producing caustic soda without the formation of undesirable chlorine by-product. These alternative methods, however, are not nearly as efficient as chlor-alkali cells. As a result, the price of caustic soda has increased several-fold.
During the last few years, chlorine dioxide has represented a commercially viable alternative for chlorine in bleaching pulp. The starting materials for chlorine dioxide are sulfuric acid, sodium chlorate, and a reducing agent, such as methanol or hydrogen peroxide. The inevitable by-product of this chlorine dioxide process is sodium sulfate (Na.sub.2 SO.sub.4), which is commercially known as salt cake. A part of the salt cake by-product is consumed by pulp mills to make sodium sulfide, a component of black liquor. However, the bulk of the salt cake does not have any use and must simply be disposed of. Salt cake is also the waste by-product of several other chemical processes.
The present inventors have now discovered a means of utilizing chlorine and salt cake waste to generate starting materials for chlor-alkali cells and chlorine dioxide generators through chemical reactions of chlorine and sodium sulfate.
It is therefore a primary objective of the present invention to provide a method of producing caustic soda using chlor-alkali cells which includes a means of utilizing the hazardous chlorine by-product generated.
It is a further objective of the present invention to provide a method of producing chlorine dioxide which includes a means of utilizing salt cake by-product.
It is still a further objective of the present invention to provide a method of recycling waste chlorine and salt cake by-products to produce chemical feeds for use in industrial processes.
It is a further objective of the present invention to provide a method of recycling waste chlorine and salt cake by-products which is environmentally safe.
It is a further objective of the present invention to provide a method of recycling waste chlorine and salt cake by-products which is economical.
It is a further objective to prevent the loss of significant capital investments in the construction of chlor-alkali plants.
These and other objectives will become apparent from the following description.